Fianna Fáil leader Micheál Martin’s rare period of political happiness, beginning at 2.20am on Sunday morning when his man claimed victory in the Carlow/Kilkenny byelection, was cut short at noon on Monday.

A visibly emotional Averil Power strode onto the Leinster House plinth to say she had been really unhappy for quite some time before announcing her shock departure from the party.

“Micheál Martin has become a leader without any followers,” was her damning departing blow.

Undeniably a leading light on the Yes side in the same-sex marriage referendum, and hugely popular with gay rights activists, Power said her former party’s “cynical and cowardly approach” to the campaign had been the “last straw”.

She said she was the only one of the Fianna Fail’s Oireachtas members who did a proper canvass for the referendum in their constituency.

Former colleagues had laughed and regarded her as “crazy” when she suggested they might wear Yes badges and distribute pro-referendum when campaigning in the byelection, she revealed.

But there was more to it than that.

Delivering a crisply-worded gift to Fianna Fáil’s political opponents, she derided the party as “simply not fit for government”.

“I don’t want to be a Fianna Fáil TD. I don’t want to be a member of a party that I no longer believe in,” she declared.

Her surprising comments set a sleepy Leinster House alight, with inhabitants speculating about Power’s motivation. “It’s never about what the person says it’s about,” one politico observed.

Power was pretty much guaranteed to get a chance to contest the General Election in Dublin Bay North, where she has been working hard on the ground for years.

But former TD Seán Haughey, son of the ex-taoiseach Charles Haughey and a close friend of Martin's, also wants to run.

If the constituency organisation selected Haughey, Power would have been added to the ticket, although her clear preference would have been to contest alone.

Councillors Deirdre Heaney and Tom Brabazon were also interested.

In normal circumstances, the loss of a Senator would not be a disaster for a party.

But these are not normal circumstances for Fianna Fáil. The party has no women TDs and Power was one of only two female Senators representing Fianna Fáil in Leinster House. She said she had been rebuffed by the party repeatedly.

The strategy she was asked to develop on improving female participation in the party had been “largely ignored” she said on Monday afternoon.

When she called for former leader Bertie Ahern to be expelled from Fianna Fáil in 2011, she was "reprimanded" by the party leader. "It would be another year before he was prepared to recommend that course of action."

Power said she had become “increasingly uncomfortable” in a party that was “out of touch with the needs and concerns of ordinary people”.

She stressed her achievements but said she had lost her faith in Fianna Fáil.

Asked what she believed the future held for the party, she replied: “I don’t know. I suppose it’s none of my business.”